What's a desktop environment? Integrated environments with environment-specific apps (like KDE and Gnome) started showing up I think in the early 2000s. That was way over a decade after GUIs started to exist in standard Unix (excluding Xerox and Apple, which had it much earlier).
You start X Windows. You start a window manager (fvwm, twm, ...), and you have windowing. You start applications (xterm, xclock, xpdf, xdvi, and various image display programs), and you can start doing work. I started using it at this point; I vaguely remember the first GUI machine I did serious work on was a DECstation (with a MIPS CPU running Ultrix, which at that point was nearly completely BSD 4.2 or 4.3), probably around 89. There were vendor-specific enhancements, for example DECWindows (on VAXes running VMS), but I think on Unix people mostly ran unmodified MIT XWindows. At the same time, other people had lots of Sun 3's, running SunOS (another lightly modified BSD) with WXindows.
A few years later came Motif and CDE. I think that was early 90s. The first time I used it was in about 91 or 92, on AIX or HPUX machines.
Note that at this time FreeBSD didn't even exist yet; the *BSD OS was still in the Berkeley distributions / OEM modified like Sun or Dec stage. Openly available versions (386BSD and BSDi) came a few years later. I don't know when XWindows on BSD started; I became aware of it around 94 or so with BSDi, and at that time I don't think they had a CDE / OpenSoftwareFoundation license yet. In practice, Linux got XWindows about the same time (I started running it in late 95 on a 386-40).
